One important application of variable resistance circuitry is in transmission circuits having a variable transmission factor; such circuits are also known as multipliers. Typically, a field effect transistor is used as variable resistor, one of the factors to be multiplied is represented by a source-drain current, another factor is represented by the source-to-drain resistance of the field effect transistor, and, according to Ohm's law, the voltage across the resistance represents the product of the two factors.
In view of the nonlinear relationship between gate voltage and source-drain resistance of a field effect transistor, a combination of two variable resistance field effect transistors has been proposed, one such field effect transistor being for the control of the transmission factor, and the other serving as a comparison resistor in a negative feedback circuit. (See Communications and Electronics, December 1964, pp. 857-858.)
It is further known to compensate for the dependence of the source-drain resistance on the applied voltage by feedback of half the drain voltage to the gate electrode. (See Elektronik 1968, No. 8, pp. 236-237.)
The known circuitry is considered to be insufficiently suitable, however, for handling video signals and, in particular, for the rapid adjustment of the amplitude of broadband signals. For example, in the circuit shown in the above-cited reference, "Elektronik", coupling between drain and gate electrodes is by means of high-resistance resistors; because of the capacitance of the gate electrode, such negative feedback is effective only for an insufficient range of the video spectrum. Reducing resistance in this known circuit improves frequency behavior but severely restricts the range of adjustment of the variable resistor, this because such a resistor is parallel to the source-drain path of the field effect transistor for those signals for which the transmission factor is to be variable.
In a different known circuit of the above-mentioned type, an attempt is made to achieve feedback of the drain voltage to the gate electrode by frequency independent means. This, however, does not contribute to the solution of the problem of the invention, namely the adjustment of amplitude of broadband signals having frequency components in essentially one and the same frequency range from 0 to, e.g., 10 MHz; in other words, the multiplication of the two broadband signals.